Method and apparatus for pumping sawdust feed in a sawdust pulping system

ABSTRACT

A method of producing chemical cellulose pulp from sawdust utilizing a static down-flow retention vessel comprising the steps of continuously pressurizing a flow of sawdust using a progressive cavity pump to produce a pressurized sawdust feed for a treatment vessel, passing the pressurized sawdust feed at super-atmospheric pressure downwardly in the treatment vessel, adding treatment white black liquor to the treatment vessel, and treating the pressurized sawdust feed in the treatment vessel with treatment white black liquor to form a treated sawdust, and discharging treated sawdust from the treatment vessel.

CROSS-RELATED APPLICATION

This application is a non-provisional application claiming priority toU.S. Provisional Patent Application No. 62/270,494 filed Dec. 21, 2015,the entirety of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Many forms of naturally occurring cellulose are used to produce chemicalpulps for the production of paper. One source of cellulose is the wastefrom saw mills, namely sawdust. The pulping of sawdust has bothadvantages and disadvantages. One advantage for using sawdust as asource of cellulose is that smaller sawdust particles are relativelyeasy to impregnate with cooking liquor. For this reason the pretreatmentsystems for the chemical pulping of sawdust are less complex than thoseused to impregnate wood chips, which are generally more difficult toimpregnate than sawdust.

One disadvantage of the chemical pulping sawdust is that sawdust can beresistant to the flow of cooking liquors. The finely divided sawdustmaterial tends to form a compact matrix when exposed to liquid flow.Exposure to this compact matrix can limit liquid's flow through thesawdust material, if not prevent liquid flow altogether. For example,because batch digesters are highly dependent upon the capability ofproviding a cooking liquor circulation through the medium being pulpedsawdust), it is difficult, if not impossible, to pulp sawdust in aconventional batch digester. Also, conventional continuous digestershave difficulty handling sawdust without incorporating some form ofspecial liquid distribution device. Conventional continuous sawdustpulping systems are known in the industry and are described in U.S. Pat.No. 3,684,651, EP 0157279, and U.S. Pat. No. 6,379,504 (all of which areincluded by reference in this document).

EP '279 introduces the use of a fluidizing, high-speed, degassing,centrifugal pump (such as an MC® brand pump manufactured by Kamyr AB asdisclosed in U.S. Pat. Nos. 4,435,193 and 4,410,337) to pump a slurry ofsawdust to a treatment step where the sawdust may be subjected tochemical treatment to produce pulp. A fluidizing, high-speed, degassing,centrifugal pump is required because the fine particles of the sawdustfeed material tend to act as a solid when in a slurry and do not filterwell. The fluidizing, high-speed, degassing, centrifugal pump has afluidizer at the inlet of the pump. The fluidizer may have spokesthrough which the sawdust slurry is fed. As the sawdust slurry passesthrough the spokes, the spokes break apart the clumps of sawdust andthereby allow the solid-like slurry of sawdust feed material to bebroken from a solid form into a fluidized suspension where there is nophase separation (e.g. no separation of the solids from the liquids).The fluidized suspension operates more like a Newtonian fluid than thenon-fluidized suspension. Gases retained in the slurry of sawdust feedmaterial must be removed from the sawdust feed material suspension. Theremoval of gases can also be accomplished by the fluidizing, high-speed,degassing, centrifugal pump. The disadvantage of the fluidizing,high-speed, degassing, centrifugal pump is the fluidizing requirement,because the fluidizer is prone to clogging. Replacing a cloggedfluidizer requires the operators to deactivate the fluidizing,high-speed, degassing, centrifugal pump, which results in loss ofproduction. Great care should generally be taken, however, to achievesufficient fluidization in order to form a pumpable suspension ofsawdust material.

Another method for processing sawdust using a conventional continuousdigester is given in U.S. Pat. No. 6,379,504. The '504 patent utilizes astatic retention vessel. A “static” retention vessel is a retentionvessel without any significant internal circulation. Internalcirculation typically includes (in conventional continuous digesters,for example) screens, conduits, pumps, heaters, and the like. Whilesteam or heated liquid may be added to the pulp in the retention vessel,to ensure that the pulp is retained at cooking temperature (althoughthat is not normally necessary), static vessels do not attempt to drawliquid uniformly through the vessel. In this manner, static retentionvessels differ from conventional batch and continuous digesters. Thisconventional method involving a static retention vessel also includes aslurry pump. The slurry pump is located after a chute where the sawdustfeed stock is diluted from an initial solids consistency of 20% to 35%at the inlet of the chute to a consistency of 10% to 15% (typicallyreferred to as a “medium consistency slurry”) at the outlet of the chutewhere an inlet to the slurry pump is positioned. Transfer of mediumconsistency slurry by means of a pump prior to cooking is not energyefficient. Typically, such pumps are limited to medium consistencyslurries of between 8% and 16% consistency. In heating such a slurry tocooking temperature, the excess liquid volume must also be heated tocooking temperature. For example, a 12% slurry contains 7.33 lbs. ofliquid per pound of fiber. In contrast, a 30% slurry contains 2.33 lbs.of liquid per pound of fiber, or less than a third of the liquid perpound of fiber. The lower consistency slurry requires additional energyto heat excess liquid to cooking temperature.

The fluidizing, high-speed, degassing, centrifugal pump of EP '279 andthe slurry pump of the '504 patent both have the disadvantages of beingpart of a multipart, complicated, energy inefficient feed systemrequiring dilution liquid be added to be within a pumpable consistency.Additionally, excessive mechanical action on sawdust slurries can bedamaging to fiber properties, and is otherwise undesirable.

U.S. Pat. No. 3,684,651 describes a pulping system for sawdust wherewashed and dewatered sawdust raw material is subjected to chemicaltreatment in a vapor phase. The method of the '651 patent involves astep where washing and impregnation of the sawdust feed material iscombined and the feed sawdust material is in a finely divided state.This method provides a simple feed system, but nevertheless a feedsystem requiring multiple screw conveyors and a rotary feeder prior tothe chemical treatment vessel.

Another common method used to continuously pulp sawdust is by usingdrag-chain type digesters, for example, an M&D-type digester as shown inFIG. 138 of Volume 5 of TAPPI's Pulp and Paper Manufacture (1989),Grace, ed. These types of digesters have an inclined vessel throughwhich sawdust is moved through the cooking liquor by means of a conveyormechanism. The method of a drag-chain type digester is simplifiedcompared to other sawdust pulping methods, especially the method of U.S.Pat. No. 3,684,651. The drag-chain type digester, however, does requirea conveyor mechanism and a rotary valve. Other related hardware requirescontinuous maintenance that makes a drag-chain type digester system,especially the rotary valve, less than ideal in most pulp mills. Therotary valve is a typical star-type feeder that inherently experiencesan unbalanced pressure load due to the large pressure difference betweenthe inlet and outlet of the valve. This load imbalance typically causessignificant wear and requires regular rebuilding of the valve.

SUMMARY OF THE INVENTION

The present disclosure avoids the limitations presented by prior artcontinuous cooking systems for sawdust, and other finely dividedcomminuted fibrous material, by first eliminating the need for a rotaryvalve as a sawdust feeder to the treatment vessel. Second, the presentdisclosure eliminates the need for diluting the sawdust feed material tomedium consistency prior to pumping the sawdust feed material to atreatment vessel.

This disclosure addresses the problems inherent in treating sawdust, orother finely divided sources of cellulose material (which is within thescope of the term “sawdust” as used in the present specification andclaims, e.g. initial cellulose particles that flow more like a powderthan like conventional wood chips), and provides for more efficientpulping, and thereby allows for less maintenance. The invention ispracticed using a progressive cavity pump attached to the feed screw atthe discharge of the receiving vessel. The progressive cavity pump hasan inlet and an outlet. The inlet is operatively connected to thereceiving vessel and the outlet is operatively connected to thetreatment vessel.

It is known to use pumps, specifically a fluidizing, high pressure,degassing, centrifugal pump or a medium slurry pump in sawdust feedingsections of a sawdust pulping system. It has, however, previously beenthought that a thick stock pump such as a progressive cavity pump couldnot be used in such a position.

A progressive cavity pump is a rotary positive displacement pumputilizing a single helical rotor within a stator within a cylindricalcasing or housing. The stator has a double helical shape and is mountedor otherwise connected to the casing or housing. As the rotor movesmaterial (in this case sawdust) in a helical motion through the statorarea within the casing or housing, the sawdust is pressed and thepressure of the sawdust is increased by the motion of the progressivecavity pump. Sawdust generally exits the progressive cavity pump at apressure higher than the entering sawdust. The sawdust also exits theprogressive cavity pump without the addition of steam as pressurizingmedium or liquid to form a slurry.

Until recent developments to progressive cavity pumps, progressivecavity pumps were small in size, which limited a progressive cavitypump's throughput capacity and thus encouraged the use of multiple pumpsoperating in series to handle the flow of material in a conventionalsawdust pulping system. Progressive cavity pumps also have pressurelimitations. Additionally, the costs associated with purchasing andmaintaining multiple progressive cavity pumps were considered to beunreasonable.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of exemplary embodiments of the disclosure, as illustratedin the accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, with emphasis instead being placed uponillustrating the disclosed embodiments.

FIG. 1 is a schematic of a conventional sawdust pulping system using arotary valve.

FIG. 2 is a side schematic view of a conventional sawdust pulping systemusing dilution and a slurry pump.

FIG. 3 is a schematic view of an exemplary embodiment of a systemaccording to the present disclosure using a progressive cavity pump.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the preferred embodiment ispresented only for illustrative and descriptive purposes and is notintended to be exhaustive or to limit the scope and spirit of theinvention. The embodiments were selected and described to best explainthe principles of the invention and its practical application. One ofordinary skill in the art will recognize that many variations can bemade to the invention disclosed in this specification without departingfrom the scope and spirit of the invention.

The present disclosure describes a feed system for a sawdust pulpingprocess where a progressive cavity pump is used in place of a rotaryfeeder to pressurize and transport sawdust feed material from areceiving vessel to a treatment vessel.

FIG. 1 is a schematic view of a conventional sawdust pulping system 100with a rotary valve 105 configured to feed sawdust 101 into the sawdustchute 122, 127. Sawdust 101 is fed to a feed conveyor 102. Recycleddigester rejects 103 from other digesters in other pulping systems mayalso be fed to feed conveyor 102. The teed conveyor 102 transportssawdust 101 and recycled digester rejects 103 to a receiving vessel 104.The receiving vessel 104 may receive exhaust steam 126. Exhaust steam126 is purge steam 112 which has been distributed to the rotary valve105 to aide in purging solid material caught in the pockets of a rotaryvalve 105. Sawdust 101 from the receiving vessel 104 is discharged to afeed screw 124 located at the discharge end of the receiving vessel 104.Sawdust 101 in the feed screw 124 moves from the receiving vessel 104into an upper section of sawdust chute 127, connecting the discharge endof the feed screw 124 and the inlet pockets of the rotary valve 105.

The rotary valve 105 is comprised of multiple pockets to receive sawdust101 feed material from the upper section of the sawdust chute 127 atatmospheric or near atmospheric pressure and pressurize the sawdust 101while within the rotary valve 105 to the operating pressure of thetreatment vessel 106. The operating pressure of the treatment vessel 106may be between 2 bar and 15 bar absolute. Purge steam 112 may bedistributed through purge steam distributor 114 to rotary valve 105 aschute purge steam 112. From rotary valve 105, pressurized sawdust 101 istransported via sawdust chute lower section 122 where treatment whiteblack liquor 119 is added as the pressurized sawdust 101 enters thetreatment vessel 106.

Black liquor 110 (source of black liquor 110 may be elsewhere in themill) may be pumped via black liquor pump 111 to a stream of whiteliquor 108 associated with white liquor pump 109. White liquor 108 andblack liquor 110 may be combined to give a combined white black liquor113. Combined white black liquor 113 is heated in a heat exchanger 107using purge steam 112 from purge steam distributor 114. It may bedesirable to bypass heat exchanger 107 with at least a portion of thecombined white black liquor 113 as unheated white black liquor 117.Heated white black liquor 118 may be mixed with at least a portion ofunheated combined white black liquor 117 to form treatment white blackliquor 119. Treatment white black liquor 119 is used in the treatmentvessel 106 to chemically treat the pressurized sawdust 101 enteringtreatment vessel 106.

Purge steam 112 from purge steam distributor 114 is typically providedto treatment vessel 106 as digester purge steam upper 115 and digesterpurge steam lower 116 to aide in the movement of sawdust 101 through thetreatment vessel 106. It is possible for only one of the digester purgesteam upper 115 or digester purge steam lower 116 to be used.

Sawdust 101 from treatment vessel 106 is discharged into surge tube 121.From surge tube 121, treated sawdust 123 may be sent for furtherprocessing, including depressurization, cooling, washing, bleaching,etc. Volatile gases are removed from treatment vessel 106 via DNCGrelease 120.

FIG. 2 shows a schematic diagram of a conventional sawdust pulpingsystem 200 with dilution and a slurry pump 10 for pulping finely dividedcomminuted cellulose material referred to as “sawdust” 101 herein. Thesawdust 101 is fed continuously by feed conveyor 11 into a receivingvessel 12 where pretreatment may take place. Pretreatment may consist ofsteaming or treatment with black liquor 110 or some other strength oryield enhancing chemical, for example polysulfide or anthraquinone andtheir derivatives. Treatment and retention in receiving vessel 12 may befrom 5 minutes to 60 minutes, but is preferably between 5 minutes and 20minutes. The receiving vessel 12 may operate at atmospheric orsuper-atmospheric pressures.

The receiving vessel 12 may exhibit single-convergence and side reliefas disclosed in U.S. Pat. No. 5,500,083 and U.S. Pat. No. 5,628,873.Receiving vessel 12 discharges into a conveyor 13 which includes aconventional conveying screw as shown in FIG. 1, or any otherconventional means of conveying the pretreated sawdust may be provided.The conveyor 13 typically comprises a screw 13′ driven by a drive devicesuch as an electric motor 13″, for example a variable speed electricmotor. If the conveyor 13 is pressurized, some form ofpressure-isolation device can be used between the receiving vessel 12and the conveyor 13. For example, a star-type feeder, such as rotaryvalve 14, may be used. The conveyor 13 is a first mixer for mixing steamand cooking liquor with the sawdust 101.

Cooking liquor, for example Kraft white liquor, is added to the conveyor13 in white liquor line 43 to begin the impregnation of the materialwith cooking chemicals. Steam may be, but is not necessarily, added tothe conveyor 13 via steam line 15 to begin the heating or continue theheating of the material begun in the vessel 12 and to remove unwantedair from the material. The conveyor 13 may also include a vent 16 forreleasing non-condensable gases (NCG) to a conventional NCG collectionsystem. A slurry having a consistency of about 25% or more and atemperature of between about 125° F. to 175° F. may be discharged fromconveyor 13.

The conveyor 13 discharges to a teed chute 17 in which the sawdust 101slurry is diluted to a consistency of between about 5% to about 15%. Thetemperature of the sawdust 101 slurry in the feed chute 17 may bebetween about 150° F. to about 250° F. The feed chute 17 feeds aconventional slurry pump 18. The slurry pump 18 pressurizes andtransfers the sawdust 101 slurry to a conventional dewatering conveyor19 via slurry conduit 20. The slurry may be diluted to lower theconsistency of about 5% to about 10% in the slurry conduit 20, e.g. bydilution liquid (e.g. recirculated liquor, filtrate, or hot water),added via dilution liquor conduit 21. The dewatering conveyor 19 may bea conventional separator such as a “top separator” or an “inverted topseparator” or another suitable conveyor.

The liquor removed from this dewatering conveyor 19, via hot liquor line22 is typically at about 250° F. to about 300° F., may be used as thesource of dilution in the dilution liquor conduit 21, after beingpressurized in pump 23 and heated in heat exchanger 26. All or part ofhot liquor in hot liquor line 22 may be flashed to produce a source ofsteam using conventional flash tank 24. For example, the pressure of thehot liquor in hot liquor line 22 may be decreased under controlledconditions, i.e. flashed, in flash tank 24 to produce a source ofcontaminated steam 25 and hot flashed liquor 25′. The contaminated steam25 may be used as the source of steam introduced to the conveyor 13 orreceiving vessel 12. This contaminated steam 25 may be supplemented byclean steam as needed. The hot flashed liquor 25′ from flash tank 24 maybe used as the source of dilution liquid in teed chute 17, or elsewhere.

The dewatering conveyor 19 increases the consistency of the sawdust 101slurry to between about 20% to about 40% and discharges the sawdust 101slurry to a conventional steam mixer 27. The steam mixer 27 may be anyconventional device (e.g. having an internal conveying screw) forintroducing steam to the slurry and heating the slurry to cookingtemperature, typically about 250° F. to about 350° F. (from 2 bar to 10bar), while the slurry's consistency is being diluted by the steamaddition to between about 15% to about 35%.

The sawdust 101 slurry discharged from the steam mixer 27 proceeds to aretention vessel/digester 28 in which the cooking reaction is allowed toproceed. The retention time in the retention vessel/digester 28 mayrange from about 30 minutes to about 6 hours. It should be noted thatretention vessel/digester 28 is static, that is, retentionvessel/digester 28 does not include any real cooking circulations orassociated screens, because cooking circulations would be difficult tooperate for such a finely comminuted material as sawdust 101. Theretention vessel/digester 28 need not include an agitator at a retentionvessel/digester discharge 29 but preferably includes as the discharge 29a non-mechanical means, such as a single-convergence outlet with siderelief as illustrated schematically in FIG. 2 or liquid discharge jetsor nozzles.

The material is discharged through discharge 29 from retentionvessel/digester 28, typically at between about 5% and about 20%consistency, and is transferred, while still at cooking temperatures andpressures (and without destructive reduction of pressure), via transferconduit 30 to a second treatment vessel 31. In the second treatmentvessel 31 the cooked, hot, pressurized material is cooled by means offiltrate from filtrate line 32. The heat of the treated materialentering second treatment vessel 31 is removed via liquid extractionline 33 and used, for example, as a source of heat for heat exchanger26. The hot liquor in liquid extraction line 33 is cooled somewhat inheat exchanger 26 and may then be sent to a conventional chemicalrecovery system, for example, to one or more flash tanks, toevaporators, to a recovery boiler, etc. The liquor in liquid extractionline 33 may also be used to treat material in receiving vessel 12,conveyor 13 or feed chute 17.

The second treatment vessel 31 may be a pressure diffuser where thecooked sawdust 101 is typically cooled by diffusing the cooler liquidfrom filtrate line 32, typically brownstock washer filtrate, through apulp bed of cooked sawdust 101 (pulp) formed in the second treatmentvessel 31. The pulp is cooled to below cooking temperature (e.g. belowabout 250° F.) in the second treatment vessel 31. The hot cooking liquoris displaced by the cooler liquid in this process and the hot displacedliquor is extracted as is conventional from the bottom of the pressurediffuser (in liquid extraction line 33). The cooled pulp is dischargedfrom the top 34 of the second treatment vessel 31 and passed by cooledmaterial discharge conduit 35 to a high density brown stock storagevessel 36 or the like. The pulp stored in the high density brown stockstorage vessel 36 may be further treated by, for example, washing orbleaching, and sent to a paper, board, or pulp machine.

FIG. 3 is a schematic view of an embodiment of a system according to thepresent disclosure using a progressive cavity pump. A sawdust pulpingsystem with a progressive cavity pump 300 has been conceived. Sawdust101 is fed to a receiving vessel 104. Sawdust 101 is discharged fromreceiving vessel 104 into feed screw 124. Connected to feed screw 124 isa thick slurry pump, specifically a progressive cavity pump 225. Theprogressive cavity pump 225 has a stator piece 226 attached to thehousing 228 and a rotor 227 within the housing 228. An inlet 229 isoperatively connected to feed screw 124 and an outlet 230 is operativelyconnected to the treatment vessel 106. In some instances, the outlet 230may be connected to a sawdust chute 122A or it may be connected directlyto treatment vessel 106. If sawdust chute 122A exists, it may be a pipeand may be short in length, such as 1 foot (“ft.”) to 2 ft. in length.If the sawdust chute 122A is used, treatment white black liquor 119 maybe added to sawdust chute 122A. It is also possible to add treatmentwhite black liquor 119′ to the outlet 230 of the progressive cavity pump225. If desired, it is also possible to add treatment white black liquor119″ directly to treatment vessel 106. If desired, any combination oflocations for treatment white black liquor 119, 119′, 119″ may be used.

When using progressive cavity pump 225, it is not necessary to providepurge steam 112 to the progressive cavity pump, but it may be desirableto add purge steam 112 to treatment vessel 106 through one or bothdigester purge steam upper 115 and digester purge steam lower 116. Anyvolatile gases produced in treatment vessel 106 are removed via DNCGrelease 120. As part of the system purge, steam 112 may first flow to asteam distributor 114 and then to treatment vessel 106.

While the invention has been particularly shown and described withreferences to exemplary embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A method of producing chemical cellulose pulpfrom sawdust utilizing a static down-flow retention vessel, comprisingthe steps of continuously: using a progressive cavity pump to pressurizea flow of sawdust to produce a pressurized sawdust feed for a treatmentvessel; passing the pressurized sawdust feed at super-atmosphericpressure downwardly in the treatment vessel; adding treatment whiteblack liquor; treating the pressurized sawdust feed in the treatmentvessel with treatment white black liquor to form a treated sawdust; anddischarging treated sawdust from the treatment vessel.
 2. The method ofclaim 1, wherein the flow of sawdust is maintained at between about 5%and about 20% consistency as said flow of sawdust moves from theprogressive cavity pump to the treatment vessel.
 3. The method of claim1, wherein the flow of sawdust is maintained at between about 20% andabout 35% consistency as said flow of sawdust moves from the progressivecavity pump to the treatment vessel.
 4. The method of claim 1, whereinthe flow of sawdust is maintained at between about 5% and about 35%consistency as said flow of sawdust moves from the progressive cavitypump to the treatment vessel.
 5. The method of claim 1, wherein thetreatment white black liquor is added to a sawdust chute.
 6. The methodof claim 1, wherein the progressive cavity pump has an outlet.
 7. Themethod of claim 6, wherein the treatment white black liquor is added tothe outlet of the progressive cavity pump.
 8. The method of claim 1,wherein the treatment white black liquor is added directly to thetreatment vessel.
 9. The method of claim 8, wherein treatment whiteblack liquor is additionally added to a sawdust chute.
 10. The method ofclaim 6, wherein treatment white black liquor is additionally added to asawdust chute.
 11. The method of claim 6, wherein treatment white blackliquor is additionally added directly to the treatment vessel.
 12. Themethod of claim 7, wherein treatment white black liquor is additionallyadded directly to the treatment vessel.
 13. The method of claim 7,wherein treatment white black liquor is additionally added to a sawdustchute.
 14. The method of claim 1, wherein no purge steam is added to theprogressive cavity pump.
 15. The method of claim 1, wherein purge steamis added to the treatment vessel through a digester purge steam upper.16. The method of claim 1, wherein purge steam is added to the treatmentvessel through a digester purge steam lower.
 17. A system for producingchemical pulp from sawdust, comprising: a receiving vessel to receive aflow of sawdust; a feed screw for transporting the flow of sawdust fromthe receiving vessel to a progressive cavity pump, wherein saidprogressive cavity pump pressurizes and transports the flow of sawdustto a treatment vessel; treatment white black liquor fed to the treatmentvessel; and treated sawdust discharged from the treatment vessel. 18.The system of claim 17, wherein the flow of sawdust is maintained atbetween about 20% and about 35% consistency as said flow of sawdustmoves from the progressive cavity pump to the treatment vessel.
 19. Thesystem of claim 17, wherein the flow of sawdust is maintained at betweenabout 5% and about 35% consistency as said flow of sawdust moves fromthe progressive cavity pump to the treatment vessel.
 20. The system ofclaim
 17. wherein treatment white black liquor is added to one or moreof: a sawdust chute, an outlet of the progressive cavity pump, or thetreatment vessel.